Squib arrangement initiated by exploding wire



Sept. 28, 1965 H. H. M KEE ETAL 3,208,379

SQUIB ARRANGEMENT INITIATED BY EXPLODING WIRE Filed Feb. 21, 1961 am w j200 as I I 34 32 4Q Q F I G. 2.

WILLIAM F GREEN HEKBEIZT H. MCKEE INVENTORS Q BY W AGENT.

United States Patent 3,208,379 SQUIB ARRANGEMENT INITIATED BY EXPLODINGWIRE Herbert H. McKee, Granada Hills, and William F. Green,

Van Nuys, Califl, assignors to Special Devices, Inc.,

San Fernando, Calif., a corporation of California Filed Feb. 21, 1961,Ser. No. 90,775 6 Claims. (Cl. 102-28) This invention relates generallyto the squib art and, more particularly, to an improved arrangement forinitiating squib operation in a secondary explosive filled squib.

The increase in utilization of relatively high direct current voltageand alternating current systems in aircraft and missiles hasnecessitated an increase in the no fire voltage level (the voltage levelat which ignition will never occur) for squibs that are incorporated inexplosive actuated devices associated with such aircraft and missiles.As a result, the always fire voltage level (the voltage level at whichan ignition will always occur) has also increased. Further, safetycriteria indicate that the use of primary explosives is not desirable inthese applications. Therefore, it has become necessary to provide asquib having initiation circuitry that will not ignite a secondaryexplosive under the relatively high direct current no fire voltages andalternating current levels and yet positively ignite under the alwaysfire voltage level. Further, it is also desirable that non-destructivetests of the squib initiation circuitry characteristics be conductedprior to utilization of the squib.

Prior arrangements have not always proven completely satisfactory inmeeting all these requirements and yet still providing reliableignition. A spark gap in the squib initiation circuitry has providedsome measure of no fire safety but the squib may be prematurely actuatedby radar energy and static electricity. In addition, a spark gap can notbe check for gap separation or circuit resistance after assembly withoutexploding the squib. Other arrangements, which have incorporated only anexploding bridge wire in the squib initiation circuitry, have often beenrendered subsequently inoperative by application of voltages which donot explode the bridge wire but only melt it.

Accordingly, it is an object of this invention to provide an improvedsquib.

It is a further object of this invention to provide a squib in whichignition is initiated by an exploding wire.

It is another object of this invention to provide an exploding wiresquib that is insentive to electrostatic or radar energy pulses. 1

It is another object of this invention to provide an exploding bridgewire squib incorporating an electric gap in the squib initiationcircuitry.

It is another object of this invention to provide an exploding bridgewire squib incorporating an electric gap in which non-destructive testsof the squib initiation circuitry properties may be made prior toutilization.

The above and other objects are realized in accordance with theprinciples of this invention by providing, in squib initiationcircuitry, an electric gap in series with an exploding bridge wiremeans. In one embodiments of this invention a pair of electricallyconductive leads extend inwardly, through a dielectric sealing means, tothe interior of a secondary explosive filled case. Each lead iselectrically isolated from the other lead and from the case. A fi st endof an exploding bridge wire is connected to a first of the leads on theinterior of the case and the bridge wire is in direct contact with thesecondary explosive. A second end of the exploding bridge wire isconnected to an elec trically conductive, thin-walled, generallycylindrical sleeve surrounding at least a portion of the second leadthat extends into the interior of the case. The sleeve is electri-3,208,379 Patented Sept. 28, 1965 cally isolated from the second leadthat it surrounds by dielectric spacers. The dielectric spacers areproportioned to provide a preselected, known gap between the sleeve andthe second lead.

Upon application of a high voltage current pulse across the pair ofleads, the gap between the sleeve and the second lead breaks downproviding circuit continuity between the leads through the bridge wire.The high voltage current pulse explodes the bridge wire and the energyassociated with the exploding bridge wire ignites the secondaryexplosive.

The invention is described in greater detail in the followingspecification taken together with the accompanying drawing, in whichlike reference characters are utilized for similar elements and inwhich:

FIGURE 1 illustrates the structures associated with one embodiment ofthis invention;

FIGURE 2 illustrates an arrangement for non-destructive circuit testingof this invention; and

FIGURE 3 illustrates another embodiment of this invention.

Referring now to FIGURE 1, there is shown a sectional view of a squibarrangement, generally designated as 10, according to the principles ofthis invention. The squib arrangement 10 comprises a generally cupshaped case means 12 in which secondary explosives 14 and 16 areenclosed. The secondary explosive 14 may take the form of a consolidatedgrain and the secondary explosive 16 may be in the form of a loose grainto allow easier ignition. Alternatively, both secondary explosive 14 andsecondary explosive 16 may be consolidated grains. Applicants have foundthat the secondary explosive PETN (pentaeryth-rite tetranitrate) hasprovided reliable operation in squib arrangements incorporating thisinvention.

The secondary explosives 14 and 16 are sealed in case means 12 by adielectric sealing means 18. Sealing between the dielectric sealingmeans 18 and case means 12 is preferably hermetic to insure that theoperation of the squib 10 is unaffected by ambient humidity and pressurevariations. This may be achieved by rolling and crimping the case means12 to the dielectric sealing means 18, as shown on FIGURE 1, or otherwell-known sealing methods may be employed. The dielectric sealing means18 may be glass, polyester resin, or the like. A pair of electricallyconductive leads 20 and 22 extend through the dielectric sealing means18 and have portions 20a and 22a, respectively, interior the case means12 and portions 20b and 22b, respectively, exterior the case means 12.The pair of leads 20 and 22 are electrically insulated from each otherand from the case means 12 by the dielectric sealing means 18.

An electrically conductive sleeve means 24 is electrically insulatedfrom and maintained in voltage breakdown gap relationship to lead 22 bythe dielectric spacers 26 and 28. The gap between the sleeve 24 and thelead 22 is selected to provide a voltage breakdown between the sleeve 24and the lead 22 at a predetermined minimum always fire voltage.

An exploding bridge wire means 30 has a first end 30a connected inelectric continuity relationship with the sleeve 24 and a second end 30bconnected in electric continuity relationship to the portion 20a of lead20.

In operation, a direct current source 32 is applied across the leads 20and 22 on their exterior portions 20b and 2211 through a low impedancegap switch 34. The voltage of the applied direct current source 32 is atleast the always fire voltage. Under the influence of this voltage,there is a breakdown of the gap between the portion 22a of the lead 22and the sleeve 24. When this voltage breakdown occurs, there is electriccontinuity between the lead 22 and the lead 20 through the explodingbridge wire means 30 and the resulting current pulse at the always firevoltage explodes the exploding bridge wire means 30 which ignites thesecondary explosive 16. The ignited secondary explosive 16 ignites thesecondary explosive 14- to provide squib operation.

The exploding bridge wire means 30 is selected so that a current pulseat the always fire voltage explodes the wire. While it is known thatwires can be made to explode on application of a current pulse at a highvoltage, the exact phenomena associated with an exploding wire reactionhas not yet been precisely identified. The book Exploding Wires, editedby William G. Chace and Howard K. Moore, Plenum Press, Inc., 1959, basedon the Conference on Exploding Wire Phenomenon, April 1959, presentsseveral theories on the underlying mechanism of exploding wires.However, it is known that the energy release accompanying an explodingwire is great enough to ignite a secondary explosive, such as PETN. Thispermits the fabrication of reliable squibs that do not require a primaryexplosive such as fulminate of mercury or lead azide in their powdertrain.

In the squibs fabricated by applicants in accordance with the principlesof this invention, the leads 20 and 22 were formed from number 22,American Wire Gage (approximately 0.02535 inch outside diameter),quadruple Formvar coated, copper wire. The sleeve means 24 was formedfrom a section of Kovar tubing, a very low thermal expansion metalmanufactured by Stupikofi Ceramic and Manufacturing Company, Latrobe,Pennsylvania, having an external diameter of 0.040 inch and an internaldiameter of 0.030 inch providing a radial gap of approximately 0.0023inch between the sleeve means 24 and the lead 22. The exploding bridgewire means was formed from two strands of 2 mil gold wire. These squibswere exposed to no fire voltages of 36 volt direct current and 110 volt,60 cycle, alternating current and the squibs were not ignited nor wasthe initiation circuitry destroyed. When exposed to an always firevoltage of 2000 volt direct current, 1 microfarad capacitance, through alow impedance gap switch, the electric gap broke down and the explodingbridge wire 30 was exploded thereby igniting the secondary explosive 16,which was composed of less than 100 mesh loose pack PETN. This, in turn,ignited the secondary explosive 14 which was composed of less than 40mesh PETN, consolidated to 15,000 pounds per square inch, therebyproviding the desired squib operation.

In some applications, reliability considerations may require that aplurality of exploding bridge wire means, connected in parallel, beincluded in the squb initiation circuitry. Thus, as in the exampleabove, two exploding bridge wires were provided between the sleeve 24and lead 20. In such a configuration, each bridge wire is selected sothat it explodes at the always fire voltage under the condition of onlyone exploded bridge wire intact and also under the condition of bothbridge wires intact.

The squibs fabricated in accordance with this invention may also bechecked, after assembly, for initiation circuitry continuity andresistance. One method of this testing is illustrated schematically inFIGURE 2. A high frequency, low voltage, signal source 36 energizes acapacitance resistance bridge 38 which has the squib 10 connected acrossone branch of the bridge. Any capacitance in the system may be nulledout by phase shift in phase and null detector 40. The high frequencysignal passes through the gap between the sleeve means 24 and lead 22 toprovide circuit continuity without exploding the exploding bridge wiremeans 30. Resistance of the initiation circuit is then read by standardbalanced bridge techniques. For the particular squib described above,applicants have found that a signal at a frequency of approximately 1megacycle and volts provides satisfactory testing.

In FIGURE 3 there is shown in partial section the structure associatedwith another embodiment of this invention. In this embodiment, a pair ofelectrically conductive leads 20 and 22' extend through a dielectricsealing means -18 to the interior of the squib 10. The leads 20 and 22'are electrically insulated from each other and also from the case means12. An interior portion 22a of the lead 22' is hollowed out to provide agenerally thin-walled section. A pair of dielectric spacers 42 and 44retain an electrically conductive pin means 46 within the hollowed outportion 22a electrically insulated from and in voltage breakdown gaprelationship to the portion 22a of the lead 22. An exploding bridge wiremeans 30 is connected between the pin means 46 and an interior portion20a of the lead 20. v

The operation of this embodiment is similar to the operation of theembodiment shown on FIGURE 1. When the always fire voltage is appliedacross the leads 20 and 22, the gap between the pin means 46 and theportion 22a of the lead 22 breaks down. The resulting current pulseexplodes the exploding bridge wire means 30 which ignites the secondaryexplosive 16. The secondary explosive 14 is thereby ignited to providethe desired squib operation.

There are many modifications and adaptations of this invention that maybe devised by those skilled in the art. For example, the voltagebreakdown gap may be provided in the squib initiation circuitry by apair of flat plates with a dielectric therebetween. In addition, acompletely redundant squib initiation circuitry may be provided, forreliability, by including two pair of leads fabricated in accordancewith the principles of this invention. Therefore, the foregoingspecification and the appended drawing are to be considered asillustrative only and the following claims are intended to cover allvariations and adaptations that do not depart from the true scope andspirit of this invention.

Having thus described a new and improved squib arrangement, what isclaimed as new and is desired to be secured by Letters Patent of theUnited States is:

1. In combination: a generally cup shaped case means; a secondaryexplosive powder train contained within said case means; a dielectricsealing means hermetically sealing said secondary explosive powder trainin said case means; a first and a second electrically conductive leadextending through said dielectric sealing means and each of said leadshaving an interior portion inside said case means and each of said leadselectrically insulated from each other and from said case means; anelectrically conductive sleeve means in voltage breakdown gaprelationship to said interior portion of said first lead and said sleevemeans electrically insulated from said first lead, and defining avoltage breakdown gap therewith for breaking down at a preselectedvoltage to transmit an electric current at said preselected voltagetherebetween; and at least one exploding bridge wire means connectedbetween said sleeve means and said interior portion of said second lead,said exploding bridge wire means in ignition relationship to saidsecondary explosive powder train, and adapted to explode upon subjectionto an electric current at said preselected voltage.

2. In combination: a generally cup shaped case means; a secondaryexplosive powder train contained within said case means; a dielectricsealing means hermetically sealing said secondary explosive powder trainin said case means; a first and a second electrically conductive leadextending through said dielectric sealing means and each of said leadshaving an interior portion inside said case means and each of said leadselectrically insulated from each other and from said case means, saidinterior portion of said first electrically conductive lead having wallsdefining a cavity; an electrically conductive pin means contained withinsaid cavity in voltage breakdown gap relationship to said interiorportion of said first lead and electrically insulated from said firstlead, and defining a voltage breakdown gap therewith for breaking downat a preselected voltage to transmit an electric current at saidpreselected voltage therebetween; and at least one exploding bridge wiremeans connected between said pin means and said interior portion of saidsecond lead, said exploding bridge wire means in ignition relationshipto said secondary explosive powder train, and adapted to explode uponsubjection to an electric current at said preselected voltage.

3. In combination: a generally cup shaped case means; a secondaryexplosive grain contained within said case means comprised of less than40 mesh PETN consolidated at 15,000 pounds per square inch; a secondaryexplosive powder contained within said case means in ignitionrelationship to said grain and comprised of less than 100 mesh PETN; apolyethylene sealing means hermetically sealing said secondary explosivegrain and said secondary explosive powder in said case means; a pair ofnumber 22, American Wire Gage, quadruple Formvar coated, copper wireleads extending through said polyethylene sealing means electricallyinsulated from each other and from said case means, and each of saidleads having an interior portion inside said case means and at least asection of said interior portion of a first of said pair of leads freeof said Formvar coating; a secton of Kovar tubing having an outsidediameter of 0.040 inch and an inside diameter of 0.030 inch insertedover said interior portion of said first of said pair of leads;dielectric spacer means for electrically insulating said Kovar tubingfrom said first lead whereby a radial voltage breakdown gap between saidsection of said interior portion of said first lead and said Kovartubing of approximately 0.0023 inch is maintained; and a pair of 2 milgold exploding bridge wires connected in parallel between said Kovartubing and said interior portion of said second of said pair of leads,said exploding bridge wires in ignition relationship to said secondaryexplosive powder.

4. A squib initiation circuit comprising, in combination: a pair ofnumber 22, American Wire Gage, quadruple Formv-ar coated, copper wireleads, a first of said pair of leads having a portion substantially freeof said Formvar coating and said pair of leads electrically insulatedfrom each other; a section of Kovar tubing having an outside diameter of0.040 inch and an inside diameter of 0.030 inch inserted over saidportion of said first of said pair of leads; a dielectric spacer meansfor electrically insulating said Kovar tubing from said first leadwhereby a radial voltage breakdown gap between said Kovar tubing andsaid portion of said first of said pair of leads of approximately 0.0023inch is maintained; and a pair of 2 mil gold exploding bridge wiresconnected in parallel between said Kovar tubing and said second of saidair of leads.

5. A squib initiation circuit contained within a squib comprising, incombination:

a pair of electrically conductive leads;

means defining a voltage breakdown gap comprising a first and a secondelectric conductor, said first electric conductor comprising anelectrically conductive rodlike member and said second electricconductor comprising an electrically conductive sleeve membersurrounding at least a portion of said rodlike member;

and a dielectric means between said first and said second electricconductors for electrically insulating said first conductor from saidsecond conductor at voltages less than at preselected voltage and forbreaking down at said preselected voltage to transmit an electriccurrent between said first and said second electric conductors;

and said first electric conductor coupled to a first of said pair ofelectrically conductive leads;

and at least one exploding bridge wire coupled between said secondelectric conductor and the other of said pair of electrically conductiveleads, said at least one exploding bridge wire for exploding uponsubjection to an electric current at a voltage other than a voltage lessthan said preselected voltage.

6. A squib initiation circuit contained within a squib comprising, incombination:

a pair of electrically conductive leads;

means defining a voltage breakdown gap comprising a first and a secondelectric conductor and said first electric conductor comprising a tubemember having walls defining a cavity and said second electric conductorcomprising a pin member having at least a portion thereof positionedwithin said cavity of said first electric conductor, and a dielectricmeans between said first and said second electric conductors W forelectrically insulating said first conductor from said second conductorat voltages less than at preselected voltage and for breaking down atsaid preselected voltage to transmit an electric current between saidfirst and said second electric conductors;

and said first electric conductor coupled to a first of said pair ofelectrically conductive leads;

and at least one exploding bridge wire coupled between said sec-0ndelectric conductor and the other of said pair of electrically conductiveleads, said at least one exploding bridge wire for exploding uponsubjection to an electric current at a voltage less than said reselectedvoltage.

References Cited by the Examiner UNITED STATES PATENTS 1,333,731 3/20Ransomer 313-131 X 2,129,003 9/38 Grant 313-139 X 2,189,741 2/40 Minton102-28 X 2,297,006 9/42 Lohman 102-702 X 2,708,877 5/55 Smits 102-7022,926,566 3/ Atkins et a1 102-702 2,981,186 4/61 Stresau 102-282,996,007 8/61 Franklin 102-28 3,002,458 10/61 Haas 102-702 3,040,6606/62 Johnston 102-702 X 3,052,185 9/62 Apstein 102-28 3,062,574 11/62Buntenbach et al 108-28 X SAMUEL FEINBERG, Primary Examiner.

1. IN COMBINATION: A GENERALLY CUP SHAPED CASE MEANS; A SECONDARYEXPLOSIVE POWDER TRAIN CONTAINED WITHIN SAID CASE MEANS; A DIELECTRICSEALING MEANS HERMETICALLY SEALING SAID SECONDARY EXPLOSIVE POWDER TRAININ SAID CASE MEANS; A FIRST AND A SECOND ELECTRICALLY CONDUCTIVE LEADEXTENDING THROUGH SAID DIELECTRIC SEALING MEANS AND EACH OF SAID LEADSHAVING AN INTERIOR PORTION INSIDE SAID CASE MEANS AND EACH OF SAID LEADSELECTRICALLY INSULATED FROM EACH OTHER AND FROM SAID MEANS; ANELECTRICALLY CONDUCTIVE SLEEVE MEANS IN VOLTAGE BREAKDOWN GAPRELATIONSHIP TO SAID INTERIOR PORTION OF SAID FIRST LEAD AND SAID SLEEVEMEANS ELECTRICALLY INSULATED FROM SAID FIRST LEAD, AND DEFINING AVOLTAGE BREAKDOWN GAP THEREWITH FOR BREAKING DOWN AT A PRESELECTEDVOLATAGE TO TRANSMIT AN ELECTRIC CURRENT OF SAID PRESELECTED VOLTAGETHEREBETWEEN; AND AT LEAST ONE XPLODING BRIDGE WIRE MEANS CONNECTEDBETWEEN SAID SLEEVE MEANS AND SAID INTERIOR PORTION OF SAID SECOND LEAD,SAID EXPLODING BRIDGE WIRE MEANS IN IGNITION RELATIONSHIP TO SAIDSECONDARY EXPLOSIVE POWDER TRAIN, AND ADAPTED TO EXPLODE UPON SUBJECTIONTO AN ELECTRIC CURRENT AT SAID PRESELECTED VOLTAGE.